We propose to investigate the electronic and conformational characteristics of the primary reactants in the biotin-mediated fixing and transfer of carbon dioxide in order to uncover the determinants of the biotin half-reaction mechanisms. Specifically, we propose to determine in a series of low-temperature, high accuracy X-ray and neutron diffraction experiments the charge density distributions of the 10 or so most probable reactants in the sequence: "CO2" + Enzyme-Biotin yields Enzyme-Biotin-CO2; Enzyme-Biotin-CO2 + Acceptor yields Enzyme-biotin Carboxyacceptor. From the experimentally determined charge densities we propose to map the electrostatic potential and the electric field for the several reactant molecules. These maps will be more accurate than those obtainable from semi-empirical MO charge densities and will rival those obtainable from ab initio calculations. With these in hand we will attempt to predict the most and least likely approaches of reactants to stabilizing and positioning reactants, the most probable protein residues involved in the studies follow naturally from our previous studies of the conformational properties of the biotin vitamers which have strongly suggested that the sulfur of biotin is a controlling element in the translocation of the N-carboxybiotinyl prosthetic group between catalytic sites. Biotin acts as a covalently linked prosthetic group for at least 10 known enzymes. These enzymes play pivotal metabolic roles in the control of fatty acid synthesis, the control of glyconeogenesis, the catabolism of amino acids, and the synthesis of purine bases. Because the biotin enzymes are architecturally and mechanistically similar, our results will be relevant to a large class of metabolic enzymes. We are attempting, in addition, to bring a new technique to the area of biochemistry. The use of experimentally derived potential surfaces and field map to describe a sequence of biological reactions will, we hope, turn out to be a very useful tool in the study of biologically important molecular interactions.